错误“C++ 需要所有声明的类型说明符”

Question

我遇到了几个错误，[E1]“C++ 需要所有声明的类型说明符”、[E2]“使用未声明的标识符”和 [E3]“需要类名或命名空间”。

#ifndef MAIN_SAVITCH_SEQUENCE_H
#define MAIN_SAVITCH_SEQUENCE_H

    template <class T>
    class sequence
    {
    public:
        // TYPEDEFS and MEMBER CONSTANTS
        typedef T value_type;
        static const int CAPACITY = 30;
        // CONSTRUCTOR
        sequence( );
        // MEMBER FUNCTIONS
        void showSequence();
        void attach(value_type entry);
        value_type current( );
        void remove_current( );
        int size( );

    private:
        value_type data[CAPACITY];
        int used;

    };
    template <class T>
[E3]        sequence::sequence( )
    {
[E2]        used = 0;
    }
    template <class value_type>
[E3]    void sequence::attach(value_type entry)
    {
        data[used++] = entry;
    }

    sequence::value_type sequence::current( )
    //replace sequence::value_type with typename value_type for your template
    {
        return data[used-1];
    }

    void sequence::remove_current( )
    {
        used--;
    }

    int sequence::size( )
    {
        return used;
    }

    void sequence::showSequence()
    {
        std::string display = "";
        for (int i = 0; i<used; i++)
            std::cout << data[i] << "\n";

    }

#endif

Answer 1

你可以简单地在模板类中定义函数

template <class T>
class sequence
{
   //...
   sequence() : used(0) // define function inside template class.
   {
   }
   //...
}

或者您需要在模板声明之外定义的每个函数之前添加template <class T> 和<T>：

template <class T>
sequence<T>::sequence( )
^^^^^^^^^^^
{
    used = 0;
}

template <class value_type>
void sequence<T>::attach(value_type entry)
     ^^^^^^^^^^^
{
    data[used++] = entry;
}

template <class T> 
typename sequence<T>::value_type sequence<T>::current( )   
         ^^^^^^^^ // 1. value_type comes from sequence template, so you need to specify sequence<T>
^^^^^^^
// 2. sequence<T>::value_type type depends on template parameter, so typename is needed
{
    return data[used-1];
}

使用方法：

sequence<int> test;
         ^^^^ // sequence is a template, you need to specify a type